It is well known that vinyl halide, and particularly vinyl chloride polymers are used quite extensively in extruded profiles, molded objects, sheets, films and coatings and it is often desirable to attempt to increase impact strengths for these polymers. One approach taken to achieve better processability has been to blend plasticizing agents into the vinyl halide polymer. However, such blending often results in decreased tensile strength, less resistance to solvents, poor high temperature properties, etc.
Alternative approaches have included incorporating minor amounts of elastomeric materials into the vinyl halide polymer. A major drawback of this technique has been that most elastomers are incompatible with vinyl halide polymers, with the result being that mixing conditions must be quite vigorous and only a very weak mechanical mixture is obtained. For instance, the mixing of any rubber into PVC resins requires very robust equipment such as a Banbury Mixer or a two-roll rubber mill. The product from such equipment cannot be easily fed to an extruder for fabrication of a final product (e.g., a rigid profile) but must be further cut or diced to a convenient size.
To alleviate this problem, elastomers such as butadiene-acrylonitrile copolymers, chlorinated or brominated butyl rubbers, etc. (see, e.g., U.S. Pat. Nos. 3,090,768 and 3,424,818), which contain functional groups capable of enhancing compatibility, have been utilized with varying degrees of success.
In an attempt to prepare mixtures of vinyl chloride polymer and butyl rubber (an elastomer lacking compatibility-enhancing functional groups), it was reported in U.S. Pat. No. 3,158,664 that although impact strengths were appreciably increased (a maximum impact strength of 14.0 ft-lb/inch of notch was reported), considerably longer periods of time were required to produce these mixtures on a mill than were required for mixtures of vinyl chloride polymers with elastomers containing compatibility-enhancing functional groups such as butadiene-acrylonitrile.
In any case, such mixing operations generally require quite high proportions of elastomers in order to attain enhanced impact strength and quantities in excess of 15% are normal. Further, such properties as stability, chemical resistance, finished appearance, and the ability to rework scrap all suffer to varying degrees. The growth of new applications and the continuous competitive nature of the industry have spotlighted the drawbacks of these methods and promoted interest in reducing their shortcomings.
The best approach to overcome the inherent disadvantages due to incompatibility in combining vinyl halide polymers with elastomers has been to graft a substantial portion of the vinyl halide polymer to an elastomeric backbone. For instance, U.S. Pat. No. 3,629,369 teaches grafting of greater than 75% of the chlorinated butyl rubber by the vinyl chloride monomer. Very energetic free radical initiators are required. By this technique, a graft blend polymer is created, with the grafted portion providing increased compatibility for the two homopolymer species. Polymers which have been employed in combination with polyvinyl chloride in preparing these graft polymers include polyethylene, copolymers of ethylene and high alpha olefins (EPR's), terpolymers of ethylene, higher alpha olefins and minor amounts of dienes (EPDM's), and chlorinated butyl rubber. See, for example, U.S. Pat. No. 3,408,424, British Pat. Nos. 1,097,020, 1,504,062, 1,070,297, and U.S. Pat. No. 3,629,369. It is apparent that the graftable substrates cited contain readily accessible graft-sites, e.g., high degree of unsaturation, or labile groups known to facilitate grafting, e.g., chlorine, thiols.
Although these graft-blend compositions show good impact strengths, they have been found to be relatively difficult to process, possibly because the elastomer becomes cross-linked during the polymerization step or the elastomer is too extensively grafted. A further well known drawback is that the relatively high degree of grafting creates excessive compatibility between the species, preventing development of optimum impact strength, and degrading other physical properties. This problem is further aggravated when residual compatibility-enhancing polar groups, such as chlorine or thiol, remain present in the grafted substrate. A graft-blended composition of vinyl chloride and chlorinated butyl rubber prepared under conditions comparable to those used in preparing the instant butyl rubber-vinyl chloride compositions gave impact strengths of 9 ft-lb/in. of notch vs 21 ft-lb/in. notch for the graft blend of vinyl chloride and butyl rubber of the instant invention.